Field of the Invention
The present invention relates to a power supply device that includes two transformers and outputs voltages obtained by converting an AC voltage of a commercial power source into different voltages by the two transformers.
Description of the Related Art
In general, for an electronic apparatus, a power supply device with a two-converter configuration which outputs voltages of two systems: a first DC voltage which is necessary for an operation of a central processing unit (CPU) or an integrated circuit (IC) (application specific integrated circuit (ASIC) etc.) for controlling an operation of the electronic apparatus; and a second DC voltage which is necessary for an operation of a motor, a solenoid, or the like, is used. Such a power supply device adopts a configuration which includes two electromagnetic transformers (hereinafter, referred to as transformers) for outputting corresponding DC voltages. The first DC voltage is about 3 V to about 5 V, and the second DC voltage is about 24 V. Accordingly, the second DC voltage is higher than the first DC voltage. The power supply device converts a DC voltage which is obtained by rectifying and smoothing an AC voltage of a commercial power source into the first DC voltage with a first switching power source (hereinafter, referred to as a control-system power source) which includes a first transformer, and outputs the first DC voltage. Meanwhile, the power supply device converts a DC voltage which is obtained by rectifying and smoothing the AC voltage of the commercial power source into the second DC voltage with a second switching power source (hereinafter, referred to as a driving-system power source) which includes a second transformer, and outputs the second DC voltage.
As such a power supply device, a power supply device which achieves power saving by stopping a driving-system power source to reduce power consumed by a driving system when an electronic apparatus is in a standby mode (power-saving mode), which is an energy-saving state, has been known (see Japanese Patent Laid-Open No. 2006-311650).
With the above-mentioned power supply device having the two-converter configuration, when a power outage occurs or a power supply cable is pulled out, the AC voltage of the commercial power source is interrupted, and the first DC voltage from the control power source to the control system drops, which may cause an unintended operation (false operation) as the electronic apparatus. For example, in the case where the AC voltage of the commercial power source drops while a motor is rotating, it is desirable that at least rotation of the motor stops before functions of a controller stop. However, if power to the controller drops (or stops) before power to the motor stops, power supply to the motor continues in a state in which the controller cannot properly control the motor, and the motor continues unintended rotation. Accordingly, in the case where a power outage occurs or a power supply cable is pulled out, it is necessary for the power supply device with the two-converter configuration to stop the driving-system power source prior to the control-system power source so that an unintended state can be avoided.
However, with the power supply device described in Japanese Patent Laid-Open No. 2006-311650, the control-system power source may stop prior to the driving-system power source, as described above. Operation of the power supply device described in Japanese Patent Laid-Open No. 2006-311650 is controlled by a control IC, and voltage needs to be supplied to drive the control IC. The power supply device will be described specifically with reference to FIG. 10. FIG. 10 illustrates a principal part of the power supply device. Reference numeral 101 denotes a control-system power source, and reference numeral 102 denotes a driving-system power source. Reference numeral 200 denotes a controller, and reference numeral 300 denotes a driving-system load. Reference numeral 103 denotes a smoothing capacitor for the control-system power source 101 and the driving-system power source 102. Reference numeral 107 denotes a control IC for controlling the control-system power source 101, and reference numeral 108 denotes a control IC for controlling the driving-system power source 102. Furthermore, an auxiliary winding 109c which is wound in a direction opposite the winding direction of a main winding 109p of a transformer 109 of the control-system power source 101 (hereinafter, referred to as flyback coupling) is provided. A voltage Vcc3, which is obtained by rectifying and smoothing a pulse voltage induced by the auxiliary winding 109c with a diode 110 and a capacitor 111, is used as a driving voltage of the control IC 107 and the control IC 108.
A voltage Vdd which is induced by the auxiliary winding 109c is substantially represented by expression (1), where, in FIG. 10, an output voltage of the control-system power source 101 is represented by Vout11, a forward voltage of a diode 112 is represented by Vfd, the number of windings of a secondary winding 109s is represented by Nss, and the number of windings of the auxiliary winding 109c is represented by Ndd.
                              V          dd                ≈                              (                                          V                                  out                  ⁢                                                                          ⁢                  11                                            +                              V                fd                                      )                    ·                                    N              dd                                      N              ss                                                          (        1        )            
The voltage Vdd is rectified and smoothed by the diode 110 and the capacitor 111, and is supplied as a power supply voltage Vcc3 of the control IC 107 and the control IC 108. The control IC 107 and the control IC 108 control a switching operation based on the power supply voltage Vcc3. At this time, in the case where the forward voltage of the diode 110 is represented by Vfd2, the power supply voltage Vcc3 is substantially represented by expression (2).
                              V                      cc            ⁢                                                  ⁢            3                          ≈                              V            dd                    -                      V                          fd              ⁢                                                          ⁢              2                                      ≈                                            (                                                V                                      out                    ⁢                                                                                  ⁢                    11                                                  +                                  V                  fd                                            )                        ·                                          N                dd                                            N                ss                                              -                      V                          fd              ⁢                                                          ⁢              2                                                          (        2        )            
Accordingly, the rectified and smoothed power supply voltage Vcc3 is substantially proportional to the output voltage Vout11 of the control-system power source 101.
With the power supply device described in Japanese Patent Laid-Open No. 2006-311650, in the case where a power outage occurs or a power supply cable is pulled out while the electronic apparatus is operating, the power supply voltage Vcc3 of the control IC 108 does not drop before the output voltage Vout11 of the control-system power source 101 drops, as represented by expression (2). Only after the output voltage Vout11 of the control-system power source 101 drops, the power supply voltage Vcc3 of the control IC 108 of the driving-system power source 102 also drops. However, even when the power supply voltage Vcc3 drops, the control IC 108 does not quickly stop and is able to continue to operate. Therefore, the driving-system power source 102 maintains an output voltage Vout22. Accordingly, the output voltage Vout11 of the control-system power source 101 first drops, power supply to a motor continues while the driving-system power source 102 is outputting the output voltage Vout22 in a state in which the motor cannot be controlled properly, and the motor thus continues unintended rotation. Alternatively, an intended operation such as sudden motion of the motor may occur.